1. Field of the Invention
The present invention relates to star-block interpolymers of conjugated diene and monovinyl aromatic monomer and preparation of the same. More particularly, the present invention relates to star-block interpolymers having the following structure:
(X-Y)n-C
wherein X represents a random copolymer block derived from conjugated diene and monovinyl aromatic monomer;
Y represents a polyconjugated diene block;
C represents a residue derived from a multifunctional lithium based initiator; and
n represents the functionality of the initiator and has a value of not less than 3;
and preparation of the same.
2. Brief Description of Art
Generally, representative block interpolymers based on butadiene, isoprene and styrene are SBS and SIS, wherein SBS is butadiene/styrene triblock copolymers(wherein B represents a polybutadiene block and S represents a polystyrene block), and SIS is isoprene/styrene triblock copolymers(wherein I represents a polyisoprene block and S represents a polystyrene block). By employing difunctional lithium based initiators and changing the charging order of butadiene, isoprene and styrene, block interpolymers having various different structures can be obtained. Such block interpolymers comprise those having the following symmetric structures: (1) S-I-B-I-S(wherein butadiene, isoprene and styrene are charged sequentially); (2) S-B-1-B-S(wherein isoprene, butadiene and styrene are charged sequentially); (3) S-I-BI-I-S(wherein butadiene and isoprene are charged simultaneously at first, and then styrene is charged); (4) S-BS-B-I-B-BS-S(wherein isoprene is charged at first, and then butadiene and styrene are charged simultaneously); (5) S-IS-I-B-I-IS-S(wherein butadiene is charged at first, and then isoprene and styrene are charged simultaneously); and (6) S-IS-I-BI-B-I-IS-S (wherein butadiene, isoprene and styrene are charged simultaneously). In the above formulae, S represents a polystyrene block, B represents a polybutadiene block, I represents a polyisoprene block, BI represents a tapered copolymer block derived from butadiene and isoprene, BS represents a tapered copolymer block derived from butadiene and styrene and IS represents a tapered copolymer block derived from isoprene and styrene.
When butadiene and styrene are charged simultaneously, only a pentablock copolymer represented by the formula S-BS-B-BS-S, which contains a tapered copolymer block derived from butadiene and styrene, can be obtained, due to the difference between the reactivity ratio of butadiene and that of styrene. If a polar modifier is charged while charging butadiene and styrene, the reactivity ratio of butadiene and that of styrene can be changed, and thus butadiene and styrene can be polymerized in a random manner, whereby obtaining a random butadiene/styrene copolymer block SBR.
Based on the above polymer design, the present inventors truly realize the chemical compounding of random conjugated diene/monovinyl aromatic monomer copolymer rubbers and polyconjugated diene rubbers, by firstly synthesizing a rubber block selected from the group consisting of a homopolymer block derived from conjugated diene, a tapered copolymer block derived from conjugated dienes, a random copolymer block derived from conjugated dienes and combinations thereof, and then synthesizing a random copolymer block derived from conjugated diene and monovinyl aromatic monomer, in the presence of a multifunctional lithium based initiator, so as to obtain star-block interpolymers having in the same molecule both said random copolymer block and said rubber block. However, the conventional method for carrying out rubber compounding is to physically blend various rubbers in an open mill or an internal mixer, and thus, the chemically compounding method employed by the present inventors is simpler and the resultant effects are superior.
Therefore, the object of the present invention is to provide novel star-block interpolymers derived from conjugated diene and monovinyl aromatic monomer, which have in the same molecule both a random copolymer block derived from conjugated diene and monovinyl aromatic monomer and a rubber block selected from the group consisting of a homopolymer block derived from conjugated diene, a tapered copolymer block derived from conjugated dienes, a random copolymer block derived from conjugated dienes and combinations thereof, and hence possess excellent properties of the both kinds of rubbers, and can be used widely as elastomeric materials, for example, for preparing tires, especially as rubbers for making tire tread having excellent properties, such a reduced rolling resistance and an improved traction characteristic. The star-block interpolymers according to the present invention can also be further functionalized, for example, hydrogenated, epoxidated, hydroxylated, carboxylated, and the like, and furthermore, can be used to modify plastic materials by blending therewith.
Another object of the present invention is to provide processes for the preparation of the above star-block interpolymers, by which one can realize the chemical compounding in situ in a single reactor instead of physically blending.
Therefore, in one aspect, the present invention relates to star-block interpolymers having the following structure:
(X-Y)n-C
wherein X represents a random copolymer block derived from conjugated diene and monovinyl aromatic monomer;
Y represents a polyconjugated diene block;
C represents a residue derived from a multifunctional lithium based initiator; and
n represents the functionality of the initiator and has a value of not less than 3.
In another aspect, the present invention relates to processes for the preparation of the present star-block interpolymers by anionic polymerization.
In still another aspect, the present invention relates to use of the present star-block interpolymers as elastomeric materials.
The present invention will be described in more detail as follows.
The term xe2x80x9cconjugated dienexe2x80x9d used in the present invention means any monomer having in its molecule conjugated double bonds, such as C4-C6 conjugated dienes. The specific examples preferably include butadiene, isoprene, 1,3-pentadiene, 1,3-hexadiene, 2,3-dimethylbutadiene, and mixtures thereof, with butadiene and isoprene being more preferred. The term xe2x80x9cmonovinyl aromatic monomerxe2x80x9d used in the present invention means any aromatic monomer carrying one vinyl substituent on its aromatic ring. The specific examples preferably include styrene or alkyl substituted styrenes such as vinyl toluene(including all isomers), xcex1-methylstyrene, 4-tert.-butylstyrene, 4-methylstyrene, 3,5-diethylstyrene, 3,5-di-n-butylstyrene, 4-n-propylstyrene, 4-dodecylstyrene and mixtures thereof, more preferably styrene, vinyl toluene, xcex1-methylstyrene and mixtures thereof, most preferably styrene.
In the star-block interpolymers according to the present invention, block X is a random copolymer block derived from conjugated diene and monovinyl aromatic monomer. It is preferred that the conjugated diene in block X is butadiene, isoprene or mixture thereof, with butadiene being more preferred. It is preferred that the monovinyl aromatic monomer in block X is styrene or alkyl substituted styrenes, with styrene being more preferred.
In block X, the content of 1,2- or 3,4-addition polymerization structure of the conjugated diene is preferably 6 to 80 percent by weight, more preferably 10 to 50 percent by weight, based on the total weight of the units derived from the conjugated diene in block X.
In block X, the content of the units derived from the monovinyl aromatic monomer is generally 10 to 50 percent by weight, preferably 15 to 35 percent by weight, based on the total weight of block X; and the content of the units derived from the conjugated diene is generally 50 to 90 percent by weight, preferably 65 to 85 percent by weight, based on the total weight of block X.
In the star-block interpolymers according to the present invention, block Y is a polyconjugated diene block selected from the group consisting of a homopolymer block derived from conjugated diene, a tapered copolymer block derived from conjugated dienes, a random copolymer block derived from conjugated dienes and combinations thereof. Preferably, block Y is a block having the following structure:
B,
I,
I-B,
B-I,
I-BR, or
I-IB-B,
wherein B represents a homopolymer block derived from butadiene, I represents a homopolymer block derived from isoprene, IB represents a tapered copolymer block derived from butadiene and isoprene, and IBR represents a random copolymer block derived from butadiene and isoprene.
Accordingly, the star-block interpolymers according to the present invention are of the following structure:
(X-B)n-C,
(X-I)n-C,
(X-B-I)n-C,
(X-I-B)n-C,
(X-IBR)n-C, or
(X-I-IB-B)n-C.
In block Y, the content of 1,2- or 3,4-addition polymerization structure of the conjugated diene is preferably from 6 to 80 percent by weight, more preferably from 6 to 55 percent by weight, most preferably from 6 to 20 percent by weight, based on the total weight of the units derived from the respective conjugated diene in block Y. When block Y contains units derived from two different conjugated dienes, for example when block Y contains homopolymer blocks derived from both butadiene and isoprene, a copolymer block derived from butadiene and isoprene, or combination thereof, especially when block Y is I-B, B-I, IBR or I-IB-B, the weight ratio of butadiene to isoprene in such a block is preferably from 10/90 to 90/10, more preferably from 30/70 to 70/30.
In the star-block interpolymers according to the present invention, the weight ratio of block X to block Y is preferably from 10/90 to 90/10, more preferably from 30/70 to 70/30.
In the star-block interpolymers according to the present invention, n has a value of not less than 3, preferably from 3 to 150, more preferably from 3 to 50, most preferably from 3 to 10.
Preferably, the star-block interpolymers according to the present invention have a number-average molecular weight(Mn) of from 5xc3x97104 to 50xc3x97104, more preferably from 10xc3x97104 to 30xc3x97104, as measured by Gel Permeation Chromatography(GPC).
The star-block interpolymers according to the present invention can be prepared by anionic polymerization processes under conventional conditions. In one preferred embodiment, the preparation process comprises the steps of:
a) anionic polymerizing one or more conjugated dienes in a non-polar hydrocarbon solvent in the presence of a multifunctional lithium based initiator and optionally, a polar modifier, until the conjugated diene(s) is polymerized completely, to form block Y; and then
b) charging a mixture of conjugated diene and monovinyl aromatic monomer into the reaction mixture obtained in step a) above, continuing the anionic polymerization in the presence of a polar modifier until the polymerization is complete, to form block X.
Depending on the specific structure of block Y, step a) of the process according to the present invention can particularly be:
(1) when block Y is B, butadiene is polymerized in step a); 2) when block Y is I, isoprene is polymerized in step a); (3) when block Y is I-B, in step a) butadiene is first polymerized completely, and then isoprene is charged and polymerized completely;
(4) when block Y is B-I, in step a) isoprene is first polymerized completely, and then butadiene is charged and polymerized completely; (5) when block Y is I-IB-B, in step a) a mixture of butadiene and isoprene is charged into a non-polar hydrocarbon solvent and polymerized completely, in the absence of any polar modifier; (6) when block Y is IBR, in step a) a mixture of butadiene and isoprene is charged into a non-polar hydrocarbon solvent and polymerized completely, in the presence of a polar modifier.
More particularly, the star-block interpolymers represented by the formula (X-B)n-C according to the present invention can be prepared as follows:
Butadiene, in an amount to provide the desired composition of the final interpolymer, is charged to a non-polar hydrocarbon solvent in a reactor, with a polar modifier being optionally charged depending on the desired microstructure of polybutadiene, and the kind and amount of the polar modifier is dependent on the desired content of 1,2-addition polymerization structure in polybutadiene block B. The total concentration of the monomers charged is from 10 to 20 percent by weight. After the stirring is started and the initiation temperature, which is generally about 30 to 80xc2x0 C., is reached, a multifunctional lithium based initiator is charged in an amount to provide the desired number-average molecular weight of the interpolymer. The multifunctional lithium based initiator can be used alone or in admixtures. After butadiene is polymerized completely, a mixture of conjugated diene and monovinyl aromatic monomer containing a polar modifier is charged to the reactor all at once, with the amounts of the conjugated diene and the monovinyl aromatic monomer being sufficient to provide the desired weight ratio of the conjugated diene and the monovinyl aromatic monomer in block X, to form block X. The purpose of use of the polar modifier is to adjust the reactivity ratio of the conjugated diene and that of the monovinyl aromatic monomer, to achieve the random copolymerization of the conjugated diene and the monovinyl aromatic monomer and to obtain the random copolymer block X. The amount of the polar modifier used shall be sufficient to make the conjugated diene and the monovinyl aromatic monomer copolymerize randomly and can be varied within a wide range depending on the kind of the polar modifier. After the conjugated diene and the monovinyl aromatic monomer are polymerized completely, the polymerization is terminated and the resulted polymer product is recovered in a conventional manner.
More particularly, the star-block interpolymers represented by the formula (X-I)n-C according to the present invention can be prepared as follows:
Isoprene, in an amount to provide the desired composition of the final interpolymer, is charged to a non-polar hydrocarbon solvent in a reactor, with a polar modifier being optionally charged depending on the desired microstructure of polyisoprene, and the kind and amount of the polar modifier is dependent on the desired content of 3,4-addition polymerization structure in polyisoprene block 1. The total concentration of the monomers charged is from 10 to 20 percent by weight. After the stirring is started and the initiation temperature, which is generally about 30 to 80xc2x0 C., is reached, a multifunctional lithium based initiator is charged in an amount to provide the desired number-average molecular weight of the interpolymer. The multifunctional lithium based initiator can be used alone or in admixtures. After isoprene is polymerized completely, a mixture of conjugated diene and monovinyl aromatic monomer containing a polar modifier is charged to the reactor all at once, with the amounts of the conjugated diene and the monovinyl aromatic monomer being sufficient to provide the desired weight ratio of the conjugated diene and the monovinyl aromatic monomer in block X, to form block X. The purpose of use of the polar modifier is to adjust the reactivity ratio of the conjugated diene and that of the monovinyl aromatic monomer, to achieve the random copolymerization of the conjugated diene and the monovinyl aromatic monomer and to obtain the random copolymer block X. The amount of the polar modifier used shall be sufficient to make the conjugated diene and the monovinyl aromatic monomer copolymerize randomly and can be varied within a wide range depending on the kind of the polar modifier. After the conjugated diene and the monovinyl aromatic monomer are polymerized completely, the polymerization is terminated and the resulted polymer product is recovered in a conventional manner.
More particularly, the star-block interpolymers represented by the formula (X-I-B)n-C according to the present invention can be prepared as follows:
Butadiene, in an amount to provide the desired composition of the final interpolymer, is charged to a non-polar hydrocarbon solvent in a reactor, with a polar modifier being optionally charged depending on the desired microstructure of polybutadiene, and the kind and amount of the polar modifier is dependent on the desired content of 1,2-addition polymerization structure in polybutadiene block B. The total concentration of the monomers charged is from 10 to 20 percent by weight. After the stirring is started and the initiation temperature, which is generally about 30 to 80xc2x0 C., is reached, a multifunctional lithium based initiator is charged in an amount to provide the desired number-average molecular weight of the interpolymer. The multifunctional lithium based initiator can be used alone or in admixtures. After complete polymerization of butadiene, isoprene in a suitable amount and optionally, a polar modifier are charged to prepare polyisoprene block I. The use of the polar modifier depends on the desired microstructure of polyisoprene, and the kind and amount of the polar modifier is dependent on the desired content of 3,4-addition polymerization structure in polyisoprene block I. After isoprene is polymerized completely, a mixture of conjugated diene and monovinyl aromatic monomer containing a polar modifier is charged to the reactor all at once, with the amounts of the conjugated diene and the monovinyl aromatic monomer being sufficient to provide the desired weight ratio of the conjugated diene and the monovinyl aromatic monomer in block X, to form block X. The purpose of use of the polar modifier is to adjust the reactivity ratio of the conjugated diene and that of the monovinyl aromatic monomer, to achieve the random copolymerization of the conjugated diene and the monovinyl aromatic monomer and to obtain the random copolymer block X. The amount of the polar modifier used shall be sufficient to make the conjugated diene and the monovinyl aromatic monomer copolymerize randomly and can be varied within a wide range depending on the kind of the polar modifier. After the conjugated diene and the monovinyl aromatic monomer are polymerized completely, the polymerization is terminated and the resulted polymer product is recovered in a conventional manner.
More particularly, the star-block interpolymers represented by the formula (X-B-I)n-C according to the present invention can be prepared as follows:
Isoprene, in an amount to provide the desired composition of the final interpolymer, is charged to a non-polar hydrocarbon solvent in a reactor, with a polar modifier being optionally charged depending on the desired microstructure of polyisoprene, and the kind and amount of the polar modifier is dependent on the desired content of 3,4-addition polymerization structure in polyisoprene I. The total concentration of the monomers charged is from 10 to 20 percent by weight. After the stirring is started and the initiation temperature, which is generally about 30 to 80xc2x0 C., is reached, a multifunctional lithium based initiator is charged in an amount to provide the desired number-average molecular weight of the interpolymer. The multifunctional lithium based initiator can be used alone or in admixtures. After complete polymerization of isoprene, butadiene in a suitable amount and optionally, a polar modifier are charged to prepare polybutadiene block B. The use of the polar modifier depends on the desired microstructure of polybutadiene, and the kind and amount of the polar modifier is dependent on the desired content of 1,2-addition polymerization structure in polybutadiene block B. After butadiene is polymerized completely, a mixture of conjugated diene and monovinyl aromatic monomer containing a polar modifier is charged to the reactor all at once, with the amounts of the conjugated diene and the monovinyl aromatic monomer being sufficient to provide the desired weight ratio of the conjugated diene and the monovinyl aromatic monomer in block X, to form block X. The purpose of use of the polar modifier is to adjust the reactivity ratio of the conjugated diene and that of the monovinyl aromatic monomer, to achieve the random copolymerization of the conjugated diene and the monovinyl aromatic monomer and to obtain the random copolymer block X. The amount of the polar modifier used shall be sufficient to make the conjugated diene and the monovinyl aromatic monomer copolymerize randomly and can be varied within a wide range depending on the kind of the polar modifier. After the conjugated diene and the monovinyl aromatic monomer are polymerized completely, the polymerization is terminated and the resulted polymer product is recovered in a conventional manner.
More particularly, the star-block interpolymers represented by the formula (X-IBR)n-C according to the present invention can be prepared as follows:
Butadiene and isoprene, in amounts respectively to provide the desired composition of the final interpolymer, are simultaneously charged to a non-polar hydrocarbon solvent in a reactor, together with a first charge of a polar modifier for ensuring the random copolymerization of butadiene and isoprene in block IBR, and the kind and amount of the polar modifier is dependent on the desired content of 1,2- or 3,4-addition polymerization structure in polybutadiene/isoprene block IBR. The total concentration of the monomers charged is from 10 to 20 percent by weight. After the stirring is started and the initiation temperature, which is generally about 30 to 80xc2x0 C., is reached, a multifunctional lithium based initiator is charged in an amount to provide the desired number-average molecular weight of the interpolymer. The multifunctional lithium based initiator can be used alone or in admixtures. After complete polymerization of butadiene and isoprene, a mixture of conjugated diene and monovinyl aromatic monomer optionally containing a second charge of a polar modifier is charged to the reactor all at once, with the amounts of the conjugated diene and the monovinyl aromatic monomer being sufficient to provide the desired weight ratio of the conjugated diene and the monovinyl aromatic monomer in block X, to form block X. The purpose of use of the polar modifier is to adjust the reactivity ratio of the conjugated diene and that of the monovinyl aromatic monomer, to achieve the random copolymerization of the conjugated diene and the monovinyl aromatic monomer and to obtain the random copolymer block X. The amount of the second charge of the polar modifier used is dependent on the amount of the first charge of the polar modifier. If the amount of the first charge of the polar modifier is sufficient to make the conjugated diene and the monovinyl aromatic monomer, which are charged subsequently, compolymerize randomly, the second charge of the polar modifier can be omitted; and to the contrary, the second charge of the polar modifier shall be charged so as to make the conjugated diene and the monovinyl aromatic monomer copolymerize randomly and its amount can be varied within a wide range depending on the kind of the polar modifier. After the conjugated diene and the monovinyl aromatic monomer are polymerized completely, the polymerization is terminated and the resulted polymer product is recovered in a conventional manner.
More particularly, the star-block interpolymers represented by the formula (X-I-IB-B)n-C according to the present invention can be prepared as follows:
Butadiene and isoprene, in amounts respectively to provide the desired composition of the final interpolymer, are simultaneously charged to a non-polar hydrocarbon solvent in a reactor, with a polar modifier being optionally charged depending on the desired microstructure of polyconjugated diene block (I-IB-B), and the kind and amount of the polar modifier is dependent on the desired content of 1,2- or 3,4-addition polymerization structure in polyconjugated diene block (I-IB-B). The total concentration of the monomers charged is from 10 to 20 percent by weight. After the stirring is started and the initiation temperature, which is generally about 30 to 80xc2x0 C., is reached, a multifunctional lithium based initiator is charged in an amount to provide the desired number-average molecular weight of the interpolymer. The multifunctional lithium based initiator can be used alone or in admixtures. After complete polymerization of butadiene and isoprene, a mixture of conjugated diene and monovinyl aromatic monomer containing a polar modifier is charged to the reactor all at once, with the amounts of the conjugated diene and the monovinyl aromatic monomer being sufficient to provide the desired weight ratio of the conjugated diene and the monovinyl aromatic monomer in block X, to form block X. The purpose of use of the polar modifier is to adjust the reactivity ratio of the conjugated diene and that of the monovinyl aromatic monomer, to achieve the random copolymerization of the conjugated diene and the monovinyl aromatic monomer and to obtain the random copolymer block X. The amount of the polar modifier used shall be sufficient to make the conjugated diene and the monovinyl aromatic monomer copolymerize randomly and can be varied within a wide range depending on the kind of the polar modifier. After the conjugated diene and the monovinyl aromatic monomer are polymerized completely, the polymerization is terminated and the resulted polymer product is recovered in a conventional manner.
The polar modifier suitable for the present process is one selected from the group consisting of oxygen-, nitrogen-, sulfur- or phosphorus-containing polar compounds, metal alkoxides and mixtures thereof. The specific examples thereof include (1) oxygen-containing compounds, such as diethyl ether, tetrahydrofuran, a compound represented by the formula R1OCH2CH2OR2(wherein R1 and R2 can be the same or different, preferably different, and represent an alkyl group having 1 to 6 carbon atoms, such as ethylene glycol dimethyl ether, ethylene glycol diethyl ether), a compound represented by the formula R1OCH2CH2OCH2CH2OR2(wherein R1 and R2 can be the same or different, preferably different, and represent an alkyl group having 1 to 6 carbon atoms, such as diethylene glycol dimethyl ether, diethylene glycol dibutyl ether), and crown ethers; (2) nitrogen-containing compounds, such as triethylamine, tetramethyl ethylenediamine(TMEDA), dipiperidinoethane(DPE), preferably TMEDA; (3) phosphorus-containing compounds, such as hexamethyl phosphoric triamide(HMPA); and (4) metal alkoxides, such as those represented by the formula ROM, wherein R represents an alkyl group having 1 to 6 carbon atoms, O represents an oxygen atom, and M represents a metal cation such as sodium cation(Na+) or potassium cation(K+), preferably potassium tert-butoxide or potassium tert-amyloxide.
The non-polar hydrocarbon solvent suitable for the present process is selected from the group consisting of non-polar aromatic hydrocarbons, non-polar aliphatic hydrocarbons and mixtures thereof. Specific examples of such solvents include benzene, toluene, ethylbenzene, xylene, pentane, hexane, heptane, octane, cyclohexane, mixed aromatic hydrocarbons such as mixed xylene, mixed aliphatic hydrocarbons such as raffinate oil, or mixtures thereof, preferably hexane, cyclohexane and raffinate oil.
The initiator useful in the present invention can be any multifunctional lithium based initiator disclosed by the prior art and can be used alone or in admixtures. The initiator is preferably selected from the group consisting of those represented by the formula RLin or T(RLi)n, wherein R represents a hydrocarbyl having 4 to 20 carbon atoms, such as an aliphatic hydrocarbyl or an aromatic hydrocarbyl, T represents a metal element selected from the group consisting of tin(Sn), silicon(Si), lead(Pb), titanium(Ti), germanium(Ge) and the like, and n represents the functionality of the initiator and has a value of not less than 3, preferably from 3 to 150, more preferably from 3 to 50, most preferably from 3 to 10. The multifunctional lithium based initiator represented by the formula RLin can be those obtained by reacting divinylbenzene(DVB) with an alkyl lithium, as mentioned in GB2124228A, U.S. Pat. No. 3,280,084, EP0573893A2, CN1197806A and the like. The multifunctional lithium based initiator can also be those represented by the formula T(RLi)n, which contain the above metal element and can be tin(Sn)-containing multifunctional lithium based initiator represented by the formula Sn(RLi)n, with R and n having the same meaning mentioned above, such as those represented by the formula Sn(RLi)4, as mentioned in CN1148053A. Further, the multifunctional lithium based initiator can be other multifunctional lithium based initiators useful for initiating the polymerization of conjugated dienes, such as butadiene and isoprene, and styrene and having a functionality of not less than 3, such as those mentioned in U.S. Pat. No. 5,262,213 and U.S. Pat. No. 5,595,951.
The amount of the initiator used depends on the desired molecular weight of the star-block interpolymers according to the present invention. In the present invention, the multifunctional lithium based initiator is preferably used in such an amount that the star-block interpolymers according to the present invention have a number-average molecular weight of 5xc3x97104 to 50xc3x97104, more preferably 10xc3x97104 to 30xc3x97104.
The polymerization reaction can generally be terminated by using a terminating agent. The terminating agent used in the present invention can be those disclosed by the prior art and useful in anionic polymerization, such as water, methanol, ethanol, isopropanol or the like.
Optionally, conventional additives and fillers, such as antioxidants, can be added into the star-block interpolymers according to the present invention, for example Irganox 1010(trade name, available from Ciba-Geigy AG, Switzerland) and Antigene BHT or 2.6.4(trade name, 2,6-di-tert-butyl-4-methylphenol, available from Sumitomo Chemical Co., Ltd., Japan).